I left as a Royal Signals Tech Sgt (T1) after 14 years, I managed to wangle a contract which I am still on two years later. I originally started as an Infrastructure Engineer but have found myself doing more and more systems engineering.
As a bit of advice to leavers; as companies IT budgets are getting tighter (especially Defence Companies) they expect more for their money. So if you plan to go into this field ensure you have knowledge across both fields.
Useful things to have - Minimum CCNA level Networking, Active Directory, Unix, Firewalls, Content Verifcation Sevices, Exchange, X400 and X500. Be flexible and willing to learn.
Not all are needed but a combination of these will certainly impress any prospective employer.